Effects of high-power microwave irradiation on tar-rich coal for realising in situ pyrolysis, fragmentation, and low-carbon utilisation of tar-rich coal

Tar-rich coal was directly combusted during power generation, resulting in a huge waste of precious oil resources and a large amount of carbon emissions. The idea of intelligent unmanned mining machine (IUMM) for in-situ fluidised mining and conversion of tar-rich coal was proposed to solve the problems of traditional mining and utilisation methods, realise efficient utilisation of tar-rich coal, and achieve net-zero CO2 emission. A key technology of IUMM is the pyrolysis and fragmentation of tar-rich coal by high-power microwave irradiation. In this study, we carried out experiments on the strength and microstructural changes of tar-rich coal under different microwave irradiation conditions using the high-power microwave irradiation testing system to explore the effects of microwave irradiation on tar-rich coal and provide a reference for the design of IUMM. The X-ray computed tomography was employed to observe and characterize the initiation, growth, connection, and spatial distribution of internal fractures of tar-rich coal under different microwave irradiation conditions. The quantitative relationship between the strength characteristics of tar-rich coal and microwave irradiation conditions was established. The results show that under a fixed microwave power or irradiation time, the overflow rate of coal gas and tar is linearly positively correlated with microwave energy E, and both the uniaxial compressive strength and brittleness of tar-rich coal have piecewise linear negative correlation with E. With the increase of E, the internal fractures grow and connect more, which are synchronised with the decrease of the uniaxial compressive strength and elastic modulus of the tar-rich coal. With equal E input, high-power microwave has a more significant effect on strength weakening, fracturing, and fragmentation of tar-rich coal. Our findings can be used for the subsequent implementation of in-situ pyrolysis and fragmentation technology of IUMM for realising the in-situ conversion, high-efficiency utilisation, and low carbon emissions of tar-rich coal.